Many applications use methods and systems for position determination, e.g. of a geodetic instrument, a vehicle or the like, which are based on global positioning systems, such as, for example, GPS, GLONASS or the European Galileo system. These Global Navigation Satellite Systems (GNSS) are based on the reception of satellite signals.
The requirements of vehicle guidance or navigation, e.g. in agricultural, mining, trucking or railroad applications, have subtle differences to those of surveying, including a much stronger requirement for continuously available positioning. However, the ability to provide continuously available positioning can be difficult.
In agricultural applications, GNSS's are used to guide a tractor, harvester or the like along a predetermined path. This guidance may take the form of mechanisms that directly control the vehicle to maintain the vehicle along the path (hereinafter auto-steering systems) or may take the form of a display to the vehicle operator to assist the operator in maintaining the vehicle along the predetermined path.
There are a number of GNSS positioning solutions with each solution utilising differing calculation methodologies. Each positioning solution has a different level of accuracy and reliability.
In vehicle guidance applications, and in particular in agricultural and mining auto-steering systems, it is desirable to have a highly accurate positioning solution. Real Time Kinematic (RTK) Carrier Phase Differential positioning is one such solution. RTK satellite navigation is a positioning calculation methodology based on the use of carrier phase measurements of the satellite signals from the GPS, GLONASS, Galileo or like systems where one or more station(s) provides real-time corrections to increase accuracy.
In practice, RTK systems use one or more base station receiver(s) and a mobile receiver on each vehicle whereby the base station broadcasts measurements of the phase of the carrier, and the mobile receiver uses the phase measurements received with those broadcast by the base station(s).
This allows the relative position of the vehicle to be calculated very accurately. However, the absolute position of the vehicle is still subject to the same absolute error as the base station.
Positioning error from lower accuracy position solutions, such as Relative Pseudorange Delta-Phase (RPDP), is typically much greater than the error from higher accuracy solutions, such as RTK, but lower accuracy solutions are often more reliable in terms of availability. Whilst the positioning errors of RPDP solutions are larger than that of RTK, they are highly auto-correlated and change relatively slowly over short periods of time.
When the higher accuracy positioning solution becomes unavailable, it is desirable to seamlessly transition to guidance using a lower accuracy positioning solution that has a greater reliability. However, as each solution has different errors, an immediate change over from one solution to the other will result in what is known as a position jump.
Clearly, a position jump in an auto-steering system will result in an abrupt course change as the positioning system will believe that the vehicle has suddenly moved position and the auto-steering system will consequently attempt to move back onto the predetermined path.